This invention relates to microwave dielectric resonator apparatus and, more particularly, to such apparatus as oscillators that are stabilized in output frequency by a dielectric resonator.
Heretofore, dielectrically stabilized microwave oscillators have been provided in which a dielectric resonator puck stores microwave energy coupled thereto by a driving circuit. This causes the puck to resonate at a microwave frequency to which the output frequency of the oscillator becomes stabilized.
It is conventional to make such known oscillators mechanically tunable to compensate for frequency drift due to aging. In that case, the dielectric resonator is generally associated with a tuning slug movable into and out of positions at which it variously interferes with external field lines of the puck which correspond to a portion of the stored microwave energy in the puck, thereby to alter the microwave resonant frequency of the puck to which the oscillator output frequency becomes stabilized. To this end, the tuning slug, the dielectric resonator puck and the driving circuit have all been enclosed in a non-hermetic housing. Typically, the tuning slug has been disposed directly over the puck at one end of a shaft threaded into an overlying wall of the housing, the other end of the shaft lying outside the housing and being slotted to receive a screwdriver by which an operator mechanically positions the slug toward and away from the top of the puck.
While the known mechanical tuning arrangement is satisfactory in dielectrically stabilized microwave oscillators where, for example, active or negative resistance elements of the puck driving circuits are individually encapsulated for environmental protection, such as Gunn diodes, it is inherently unsatisfactory in dielectrically stabilized microwave oscillators having puck driving circuits which employ active elements which are not individually encapsulated, notably GaAsFETs, and which must be hermetically housed for environmental protection. Such driving circuits may even include buffer amplifier stages and other circuitry having exposed semiconductor devices which, independently of whether the active elements individually are encapsulated or unencapsulated, require hermetic protection from the environment.
The reason why the known mechanical tuning arrangement is inherently unsatisfactory where hermetic protection from the environment is required is simply that the known arrangement renders it virtually impossible to preserve the required hermetic protection for any considerable length of time due to contaminating leakage at the threaded shaft of the tuning slug. While a mechanical positioning of the tuning slug with a bellows arrangement would overcome the leakage problem, it would introduce intolerable mechanical hysteresis and be intolerably microphonic. Thus, up to now, mechanical tuning of hermetically sealed dielectrically stabilized microwave oscillators has not been feasible, and electronic tuning, such as by Varactors, has been the only practical way to achieve tuning access into a hermetically sealed enclosure.